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Virulence
Virulence is, by MeSH definition, the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its virulence factors.〔(MeSH - Medical Subject Headings, Karolinska Institute, 13 April 2010 )〕 The noun ''virulence'' derives from the adjective ''virulent''. ''Virulent'' can describe either disease severity or a pathogen's infectivity.〔Compact Oxford English Dictionary (virulent )〕 The word ''virulent'' derives from the Latin word ''virulentus'', meaning "a poisoned wound" or "full of poison."〔〔A Latin Dictionary (virulentus )〕
In an ecological context, virulence can be defined as the host's parasite-induced loss of fitness. Virulence can be understood in terms of proximate causes—those specific traits of the pathogen that help make the host ill—and ultimate causes—the evolutionary pressures that lead to virulent traits occurring in a pathogen strain.〔Encyclopædia Britannica Online, 25 May 2009. ("plant disease development" )〕
==Virulent bacteria==
The ability of bacteria to cause disease is described in terms of the number of infecting bacteria, the route of entry into the body, the effects of host defense mechanisms, and intrinsic characteristics of the bacteria called virulence factors. Many virulence factors are so-called effector proteins that are injected into the host cells by special secretion machines such as the type 3 secretion system. Host-mediated pathogenesis is often important because the host can respond aggressively to infection with the result that host defense mechanisms do damage to host tissues while the infection is being countered.
The virulence factors of bacteria are typically proteins or other molecules that are synthesized by enzymes. These proteins are coded for by genes in chromosomal DNA, bacteriophage DNA or plasmids. Certain bacteria employ mobile genetic elements and horizontal gene transfer. Therefore, strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed. Bacteria use quorum sensing to synchronise release of the molecules. These are all proximate causes of morbidity in the host.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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